What do Rayleigh and Nusselt numbers tell us about heat transfer?

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SUMMARY

The Rayleigh number and Nusselt number are critical parameters in understanding heat transfer through convection. The Rayleigh number indicates the onset and intensity of natural convection, with a critical value of approximately 1700 marking this transition. The Nusselt number quantifies the efficiency of convective heat transfer relative to conduction at a solid-fluid interface, influenced by Reynolds (Re) and Prandtl (Pr) numbers in forced convection, and Grashof (Gr) and Prandtl numbers in natural convection.

PREREQUISITES
  • Understanding of Rayleigh number and its significance in natural convection.
  • Familiarity with Nusselt number and its role in heat transfer efficiency.
  • Knowledge of Reynolds number (Re) and Prandtl number (Pr) in forced convection scenarios.
  • Awareness of Grashof number (Gr) and its application in natural convection contexts.
NEXT STEPS
  • Research the critical value of the Rayleigh number and its implications for natural convection.
  • Explore the relationship between Nusselt number and heat transfer coefficients in various fluid dynamics scenarios.
  • Study the effects of Reynolds and Prandtl numbers on convective heat transfer in forced convection.
  • Investigate the role of Grashof number in determining natural convection characteristics.
USEFUL FOR

Engineers, physicists, and students in thermal dynamics or fluid mechanics seeking to deepen their understanding of heat transfer mechanisms in convection processes.

billiards
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Hi

Can anyone tell me anything about these numbers. I know the formulae I just want a qualitative description.

As far as I have it the Rayleigh number tells you if you can have convection and how vigorous that convection is, and the Nusselt number tells you how efficient that convection is in transporting heat.

I could well be wrong...

Cheers
 
Science news on Phys.org
Check out the following reference:

Natural Convection in Enclosures
Book: Free-Convective Heat Transfer
Publisher: Springer Berlin Heidelberg
Pages: 279-343
 
sorry, I had a brain fart ...

Here's something I compiled for work:

Rayleigh Number:
Important in natural (free) convection
Onset of natural convection occurs when the Rayleigh number reaches a critical value (~1700)

Nusselt number:
Ratio of convective to conductive heat transfer across a solid-fluid interface
Used to calculate heat transfer coefficient at the interface
Depends upon Re and Pr in forced convection
Depends upon Gr and Pr in natural convection
Sometimes also interpreted as the dimensionless temperature gradient at the surface.
 

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